The Blech experiment

The Blech experiment illustrates the relationship between interconnect length and the rate of electromigration-induced drift.

The test structure consists of aluminium (Al) segments with varying length on the same underlying titanuim nitride (TiN) layer. These test structures allow for a direct measurement of the electromigration displacement with time.

When passing an electrical current through such a structure, the low resistivity Al will drift on top of the TiN in the direction of electron flow - material accumulation at the anode and material depletion at the cathode end.

Test structure
The TiN layer is stacked onto a SiO₂ substrate. It is able to carry an electrical current as it is a conductor. TiN has a low electrical conductivity compared to Al.

Apply potential difference

An Al segment is then stacked above the TiN layer. As Al is a better conductor than TiN, it is more favourable for the electric current to flow through the Al segment.

Apply potential difference

The Blech length

During electromigration, a stress gradient develops within the Al segment. This is a result of mass atomic motion, resulting in regions of hydrostatic tension and compression.

The critical stress gradient, which can be withstood before failure, differs for Al segments of different lengths, following the relationship:
     (jl)_c = ΔσΩ/Z*eρ

The stress gradient increases until it reaches a critical limit. If the stress gradient does not exceed this critical value, no damage will be observed.

Therefore, the shorter line will have infinite life, as its critical stress gradient can never be achieved for the given current density.

 

Electromigration Damage

If the Al segment exceeds the critical length (Blech length), electromigration-induced damage will be observed.

The damage is observed as a formation of voids and hillocks.